Talk at the APS March Meeting March 16-20 in Los Angeles, CA The application of confocal fluorescence correlation spectroscopy FCS (Magde et al. 1972, Appl. Phys. Lett. 29:705) to biologically active molecules allows highly sensitive dynamic measurements of diffusion coefficients, flow rates and kinetic rate constants. Intracellular FCS measurements of protein or nucleic acid dynamics and interactions in live cells are most promising because of the low required concentrations and minimal interference with biological functionability. Reduced signal-to-noise ratios and the risk of photodamage in cellular systems require a careful consideration of the photophysical system and evidence is given that multiphoton-excitation (Denk et al. 1990, Science 248:73) of the selected dyes such as green fluorescent protein (GFP) may be the preferable choice. Our study compares the different excitation alternatives in FCS measurements with respect to their suitability for intracellular investigations.